1. Field of the Invention
The present invention relates to RF and microwave antennas, and more particularly, to a small planar antenna and a small conductive strip radiator with improved bandwidth.
2. Description of the Related Art
In L-frequency bandwidth and at UHF frequencies, the size of a half wave dipole antenna presents a restriction in mobile or RFID applications, and therefore, a small antenna with relatively small wavelength is required. However, the size of antenna for a given application is not related mainly to the technology used, but is defined by well-known laws of physics. Namely, the antenna size with respect to the wavelength is the parameter that has the most significant influence on the radiation characteristics of the antenna.
Every antenna is used to transform a guided wave into a radiated one, and vice versa. Basically, to perform this transformation efficiently, the antenna size should be of the order of a half wavelength or larger. Of course, an antenna may be smaller than this size, but bandwidth, gain, and efficiency will decrease. Accordingly, the art of antenna miniaturization is always an art of compromise among size, bandwidth, and efficiency.
In the case of planar antennas, a good compromise may be obtained when most of the given antenna area participates in radiation.
WO 03/094293 discloses an example of miniaturizing the antenna to a size smaller than the size of resonance, while maintaining relatively high gain and efficiency of resonance characteristics. FIG. 1 shows an antenna of WO 03/094293, which is incorporated herein by reference.
Referring to FIG. 1, antenna 1 includes a dielectric substrate 2, a feed line 5, a metal layer 3, a main slot 4 and a plurality of sub slots 6a to 6d which are patterned within the metal layer 3. The metal layer 3 with the main slot 4 and sub slots 6a to 6d form a radiator of the antenna 1.
Meanwhile, FIG. 2 shows a radiator of a conventional antenna which has a vertically-linear slot. FIG. 3 shows a radiator of a conventional antenna with vertically-rotating slot, and FIG. 4 shows a radiator of a conventional antenna with a vertically-spiral slot.
Throughout the description with reference to FIGS. 2 to 4, the common components, that is, main slot and metal layer will be referred to by the same reference numerals. A plurality of sub slots 8a to 8d, 9a to 9d, 10a to 10d of various configurations, are formed at each end of the main slot 4.
A conventional antenna as exemplified above is limited by having narrow bandwidth. Furthermore, the operative frequency bandwidth of a small antenna is a factor in a variety of applications.
Accordingly a need arises for a small antenna, which can operate at an electrically-improved bandwidth, without affecting radiation pattern, gain and radiation efficiency.
Meanwhile, a small antenna requires a large amount of conductive material for a ground layer. Thus, the relatively high weight of conductive material required in antennas also becomes a factor.